1. Field of the Invention
The invention relates in general to a RC oscillator circuit, and more particularly, to a RC oscillator circuit with a stable output frequency.
2. Description of the Related Art
The RC oscillator circuit has been very broadly applied in the modern technique. In FIG. 1, a conventional RC oscillator circuit includes an oscillator 102, a resistor 104, and a capacitor 106. The oscillator 102 generates a pulse under an operation voltage. Such pulse generates a RC circuit comprised of the resistor 104 and the capacitor 106. The RC circuit has a time constant, according to which the pulse is attenuated. The oscillator 102 has an input terminal coupled between the resistor 104 and the capacitor to receive the attenuated pulse. When the voltage level of the attenuated pulse is less than a critical point, the oscillator 102 generates another pulse. According to the same mechanism, a series of pulse signals is generated with an output frequency. Based on the above mechanism, the output frequency is affected by the resistor 104 and the capacitor 106. Thus, the period or frequency of the pulse signal is changed as the design of RC time constant of the RC circuit to generate an operation frequency.
The output operation frequency of the conventional RC oscillator circuit is changed according to operation voltage of the oscillator 102. The typical operation voltage is the chip operation voltage of 3V or 5V. When the operation voltage is higher, the output operation frequency is higher. That is, the change of operation frequency is related to the operation voltage. Thus, the operation frequency is different under different operation voltage.
In the practical operation, the same output frequency is required when the RC circuit is operated under different operation voltage. However, the conventional circuit has the output frequency increases as the operation voltage. Once the oscillator circuit is operated under different operation voltage, the output frequency is changed. In the conventional oscillator circuit, the output frequency is inversely proportional to the RC time constant. When the output frequency has to be the same, the resistance or capacitance has to be changed to change the RC time constant, so as to adjust the output frequency to be the same. Such application is very complex. When the resistor and the capacitor is invariable, that is, when the resistance or the capacitance is invariable, the oscillator circuit has to be designed with different set of resistor and capacitor. Thus, in the conventional circuit, a certain set of resistor and capacitor is required for the oscillator circuit is under a predetermined operation voltage to obtain the predetermined RC time constant. Such design is very inconvenient for practical application.
The invention provides a RC oscillator circuit with a stable output frequency to resolve the problem of generating different output frequency due to different operation voltage.
The RC oscillator circuit with a stable output frequency includes the same set of resistor and capacitor to output the same frequency under different operation voltages.
The oscillator circuit with a stable output frequency includes an oscillating integrated device, a resistor, a first capacitor, a second capacitor and a MOS transistor. The oscillating integrated circuit works under an operation voltage to generate a pulse, so as to drive the resistor. The first and second capacitors construct a RC circuit. The resistor has a first terminal and a second terminal coupled to the output and input terminals of the oscillating integrated device, respectively. The first capacitor has a first terminal coupled to the input terminal of the oscillating integrated device and a second terminal coupled to the second terminal of the resistor. The second capacitor has a first terminal coupled to the second terminal of the first capacitor and a second terminal coupled to a ground voltage. The transistor has a gate coupled to the operation voltage, a drain coupled to the first terminal of the second capacitor and a source coupled to ground.
The above operation voltage includes a voltage of about 3V or 5V.
The above oscillating integrated device includes an oscillator.
The above oscillating integrated device includes a pulse generator.
The above MOS transistor is built in the oscillating integrated device.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.